There are many techniques described in the prior art for the preparation of alumina. Apparently, however, the source of aluminum values and/or the technique of treatment, such as drying, results in different forms of alumina which possess different physical and catalytic properties. Many of the aluminas, for example, are used as supports for various types of catalysts used in the petroleum and other industries. For example, it is known that aluminas derived from a byproduct of the Ziegler alcohol synthesis reaction are particularly pure and are therefore useful as a support for reforming catalysts (U.S. Pat. No. 3,852,190 to Buss et al.). Buss et al confirm or at least suggest that a variety of aluminas can be made, and the most frequently preferred alumina for use in reforming catalysts is gamma-alumina (see Col. 2, lines 41-42 of the Buss et al patent). W. B. Carter in U.S. Pat. No. 3,264,063 teaches the preparation of eta-alumina from aluminum alcoholates by introducing the aluminum alcoholates to the surface of a body of water, e.g. by spraying. On the other hand, R. L. Poe in U.S. Pat. No. 3,309,416 teaches the preparation of beta-alumina from the hydrolysis of aluminum trialkoxide. Poe also teaches in Column 4, lines 18 et seq., that the alumina hydrate produced by the hydrolysis of the aluminum trialkoxides is "yielded through line 31". Line 31 refers to the Figure in the Poe et al. reference, but no technique is taught for the recovery of the alumina hydrate from the material removed through Poe's line 31, and no teachings are present indicating any special problems associated with the alumina hydrate. J. J. Hagan et al. in U.S. Pat. No. 3,751,518 are also concerned, like Poe, with olefin production via a growth-displacement reaction in the presence of organo-metallic catalysts, and teach in Column 7 that the catalyst in the product may be deactivated by contact with acid, base, water, or alcohol, but dilute caustic is preferred. This generally follows the earlier teachings of H. B. Fernald et al in U.S. Pat. No. 3,482,000, but more especially in Fernald et al.'s U.S. Pat. No. 3,477,813. In the '813 patent in Column 5, Fernald et al. teach away from the use of plain water for the conversion of the aluminum alkyls to hydrated oxides, as the plain water tends to cause the hydrated aluminum oxides to precipitate upon the walls of said zone and in transfer lines and instrument lines, etc. Because of this problem, Fernald et al teach it is preferred to maintain the aluminum in solution in a first aluminum-removal zone by reaction with caustic so as to avoid the precipitation problems in the transfer line, etc. In Column 2, lines 38 et seq., Fernald et al. caution that if too much caustic is added, the excessive water introduced prevents rapid settling of alumina oxides later in the process.
It appears obvious that the reason why Fernald et al. were having problems with precipitation in the transfer lines, etc., was because of the slow rate at which the hydrated aluminum oxides tended to settle out from the aqueous product solution. This has been found to be true in the experimental work to be discussed below, but it has also been found in accordance with the present invention that the addition of small amounts of a particularly defined anionic form of polyacrylamide tend to result in a rapid settling of substantially all of the finely dispersed colloidal size hydrated alumina oxide particles from the ethylene growth-displacement product using an aluminum alkyl type catalyst when such reaction product is contacted with plain water.
In accordance with the invention, the aluminum values are rapidly and substantially completely removed from the product stream of an alpha-olefin process wherein ethylene is telomerized to alpha-olefins in the presence of an organo-aluminum catalyst to produce the product stream containing said organo-aluminum catalyst in the alpha-olefins. The process of this invention comprises mixing water with said product stream in an amount sufficient to produce (1) an aqueous phase containing a fine colloidal suspension of hydrated aluminum oxide particles and (2) an organic alpha-olefin phase substantially free of aluminum values. The aqueous and organic phases are suitably separated, and to the aqueous phase is added a sufficient amount of an aqueous solution of a polyacrylamide to result in at least 90 weight percent of the hydrated aluminum oxide settling in less than 10 minutes at ambient conditions. The polyacrylamide has the formula: ##STR1## where R can be hydrogen or a hydrocarbon group having from 1 to 6 carbon atoms; n is from 80.times. 10.sup.3 to 15.times. 10.sup.5 ; and wherein a sufficient portion of the NH.sub.2 groups are coverted to an acid salt form to impart anionic properties to the polyacrylamide. Preferably two aluminum removal zones are employed in a continuous process as will be described below.
Ethylene is telomerized to normal alpha-olefins having between about 4 and 40 carbon atoms in the presence of an organo-aluminum catalyst, such as triethyl aluminum, which is charged to the process in a catalyst solvent. The reaction temperature can be between about 180.degree. and 240.degree. C.; the reaction pressure is at least about 1000 pounds per square inch; there is between about 1.times. 10.sup.-.sup.4 and 1.times. 10.sup.-.sup.2 moles of catalyst per mole of ethylene, and the polymerization proceeds until there is a conversion of about 30 to 60 percent of said ethylene to polymer product. These process conditions are illustrative only and are not per se a part of the present invention. Further details of a suitable process for producing alpha-olefins can be found in U.S. Pat. No. 3,482,000 issued Dec. 2, 1969, the teachings of which are incorporated herein by reference.
In general, the product from the alpha-olefin process, disregarding catalyst solvent, comprises between about 10 and 75 weight percent unreacted ethylene, the remainder being alpha-olefin product, and between about 0.2 to about 4 weight percent of organo-aluminum catalyst having three alkyl groups with each group having an average of about 8 carbon atoms. For example, the product from the alpha-olefin process commonly comprises about 48 to 50 weight percent unreacted ethylene, about 48 to 50 weight percent alpha-olefin product and about 2 weight percent of organo-aluminum catalyst. The product from the alpha-olefin process is generally at a pressure between about 1500 and 4000 pounds per square inch or higher. It is important to substantially completely remove all the aluminum prior to charging the alpha-olefin product to a distillation column for the fractionation thereof. The presence of aluminum under distillation conditions will seriously degrade the alpha-olefin product and be generally deleterious to the distillation operation. This invention relates to a highly advantageous method for the removal of the aluminum from the product prior to charging said product to a distillation zone.
In accordance with this invention, the aluminum is removed from alpha-olefin product in a continuous process employing two aluminum-separation zones. Just prior to the first aluminum-separation zone, some ethylene gas is vented from the alpha-olefin reactor upwardly to reduce the pressure to between 50 and 1000 psig; and a mixture comprising unreacted ethylene, alpha-olefin product, catalyst solvent and organo-aluminum catalyst is admixed with the desired amount of water in a mixing tank.
In the first aluminum-separation zone, separate organic and aqueous layers form. The aqueous layer comprises a colloidal suspension of alumina particles. (The terms "alumina", "hydrated aluminum oxide" and "aluminum hydroxide" are used interchangeably in this application.) Gaseous ethylene is vented from the first zone so that the organic layer comprises the liquid alpha-olefin product together with some emulsified water.
If too little water is used, complete removal of aluminum is difficult. If too much water is used, unnecessarily large volumes must be handled which would needlessly increase the expense of the operation. Thus the amount of water should be sufficient to result in the production of an aqueous phase containing a fine colloidal suspension of hydrated aluminum oxide particles and an organic phase containing the alpha-olefin and being substantially free of aluminum values.
The volume ratio of water to the alpha-olefin reaction product can suitably be from 0.25:1 to 2:1, preferably 0.5:1 to 1:1.
The contacting is done at temperature conditions which do not result in vaporization of the higher alpha-olefin products, or, of course, the water. The temperature must be high enough, however, to maintain the alpha-olefin product in the liquid phase since the alpha-olefin product does tend to solidify to a wax-like mass on cooling to room temperature. Normally, temperatures of 40.degree. to 85.degree. C., preferably 50.degree. to 75.degree. C., are employed.
The aqueous colloidal suspension of alumina is generally and conveniently removed to a second aluminum-separation zone in order to maintain a continuous form of operation. If the colloidal suspension of alumina were allowed to settle of its own accord, inordinate and unacceptable amounts of time would be required from a commercial standpoint in order to result in substantially complete settling of the alumina to a filterable form.
As noted earlier, Fernald et al in Column 2, lines 36 to 38 of U.S. Pat. No. 3,477,813, teach that excessive water introduced with their caustic will prevent rapid settling of aluminum hydroxide in the process. Indeed times on the order of weeks might be required in order to obtain substantially complete settling of the aluminum hydroxide, if at all. There are a number of coagulant or precipitation aids on the market which might be useful in coagulating colloidal particles such as are present in the aqueous stream described above. There is no known way of predicting ahead of time whether or which of the coagulant aids which are available will function in any particular environment. A number of coagulant aids were tried with the aqueous product stream described above, but only an anionic form of a polyacrylamide to be described below was found not only to coagulate the colloidal aluminum particles but also to result in a rapid settling of the alumina particles. By a "rapid settling" is meant that over 90 percent of the colloidal alumina had settled out in a period of time of less than 10 minutes. The savings in caustic alone over the technique described by Fernald et al. and others in the prior art (in addition to avoiding the pollution problems associated with the use of caustic) constitute extra and added benefits to the process of this invention. In addition, as will be described below, extremely minute quantities of the polyacrylamide have been found sufficient to perform the coagulation of the colloidal alumina particles.
Thus in accordance with the invention, it has been found that an aqueous solution of certain high molecular weight polyacrylamides in anionic form possess unexpectedly superior properties not only in coagulating the alumina from an ethylene-growth displacement reaction product containing an organo-aluminum catalyst, but also in substantially completely settling the alumina in periods of time of less than 10 minutes. The polyacrylamide has the general formula: ##STR2## where R can be hydrogen or a hydrocarbon group having from 1 to 6 carbon atoms; n is from 80.times. 10.sup.3 to 15.times. 10.sup.5 ; and wherein a sufficient portion of the NH.sub.2 groups are converted to an acid salt form to impart anionic properties to the polyacrylamide.
The polyacrylamide suitably has a molecular weight of from one million to 100 million, which can be measured by light scattering, viscosity or osmotic techniques; and usually the molecular weight is from six million to 10 million.
The preparation of the polyacrylamides forms no part of this invention, for the preparation of these materials is well known to those having ordinarily skill in the art. For example, a suitable description of acrylamide polymers and their method of preparation can be found in the Encyclopedia of Polymer Science and Technology, Volume I, pages 177-195, published by Interscience Publishers, Div. of John Wiley & Sons, Inc., and other references contained therein.
The polyacrylamides which are useful as coagulating agents in the process of this invention have a sufficient portion of the NH.sub.2 groups converted to an acid salt form so as to impart anionic properties to the polyacrylamide. Suitably from 5 to 25 mole percent of the NH.sub.2 groups are converted. Preferably about 15 percent of the amine groups are converted to the acid salt form. The conversion of the amine groups to an acid salt form can be achieved by techniques which are well known to those having ordinary skill in the art. For example, the polyacrylamide can be reacted with the desired amount of a strong inorganic acid such as sulfuric acid to form ammonium sulfate and to convert the desired amount of amide linkages to acid linkages. Polyacrylamides made in aqueous solution are likely to have undergone appreciable hydrolysis of amide groups to carboxyl groups. This hydrolysis can be enhanced by adding the corresponding amount of sodium hydroxide (or other basic hydroxides) during polymerization. The resultant polymer thus contains Na.sup.+ (or other cations) to charge balance the carboxylate anions of the polymer chain. The term " anionic polymer" is used to describe such a polymer whereas a cationic polymer is one where the polymer chain is cationic with anions, such as fluoride, hydroxide, borate, etc., used to charge balance the polymer chain. Such materials are described in U.S. Pat. No. 3,288,770. The methods of preparing the polyacrylamides are disclosed, for example, in U.S. Pat. No. 2,820,777.
The polyacrylamides described above, even in the anionic form, have a very high molecular weight and are soluble in water, but only to a modest extent. Solutions having a concentration of 0.5 weight percent polymer can be prepared; however, above 0.2 weight percent the viscosity becomes too great for convenient use. The preferred concentrations are in the range of 0.01 to 0.15 weight percent.
It is necessary that an aqueous solution of the polyacrylamide be used, for the undissolved polyacrylamide will have little, if any, effect as a coagulating agent. It is believed that the polyacrylamide functions as a coagulant by stringing out the long molecules of the polyacrylamide in water and literally dragging the alumina particles down to the bottom of the settling vessel. It is surprising that only a small but nonetheless definite amount of the polyacrylamide is required to provide this result. For example, it has been found that amounts as little as 1-2 ppm of the polyacrylamide in the total aqueous suspension of alumina particles are sufficient to result in substantially complete settling of the alumina in very short periods of time, on the order of five to ten minutes. Amounts as low as 0.4 ppm are not suitable. The range of concentration of polyacrylamide can therefore suitably be from 0.5 to 10 ppm but is more usually from 1 to 5 ppm. Larger amounts of the polyacrylamide can be employed, for example, 100 ppm or more, but serve no useful purpose. Because of the variation in molecular weight and amide group conversion, those with ordinary skill in the art can with very little experimentation determine the proper amount of anionic polyacrylamide to use to result in settling of at least 90 weight percent of the alumina in less than 10 minutes at ambient conditions.